1. Field of the Invention
The present invention generally relates to the processing of television signals and, more particularly, to the processing of color television signals.
2. State of the Art
Generally speaking, television pictures are comprised of snapshot-like "frames" containing video data organized in horizontal lines by synchronizing signals in a manner suitable for reproduction at receivers. For example, according to the standards of the National Television Systems Committee (NTSC), each frame of video information comprises 525 horizontal scanning lines and the frame repetition rate is thirty frames per second. (Thus, the horizontal scanning line repetition rate within an NTSC frame is 15,750 lines per second and the duration of each horizontal scanning line is 63.5 microseconds.)
In practice, complete frames of video information are not reproduced at a receiver in a single cycle. Instead, to reduce flicker, a technique known as interlaced scanning is used. According to this technique, each complete frame of video information is divided into two interlaced fields, each comprising a sequence of the odd numbered or even numbered horizontal scanning lines. Thus, if the horizontal scanning lines of a 525-line NTSC frame were numbered sequentially from the top of a raster array, an odd-line field would comprise numbered lines 1, 3, 5, and so forth through frame line 525 and an even-line field would include numbered lines 2, 4, and so forth through frame line 524. For the NTSC format, the field repetition rate is sixty fields per second while the scanning line repetition rate remains at 15,750 lines per second. In practice, line numbers are based upon the publication "Recommendations and Reports of the CCIR", 1986, Vol. XI - Part 1, Rep. 624-3, pages 22-24.
The synchronizing signals that organize video information for reproduction are normally referred to as composite sync. In practice, composite sync includes vertical sync pulses which define the beginning of each video field and horizontal sync pulses which define the beginning of each horizontal line in a field. Also, the composite sync includes pre-equalizing signals.
In color television systems, each horizontal scanning line carries a sinusoidal synchronizing signal referred to as a color burst. In the NTSC standard, for example, each color burst has a frequency of about 3.58 megahertz. Color burst signals determine the color phase, or picture hue, of signals that follow the burst in the horizontal scanning line. The relationships between the frequency of the chrominance subcarrier (Fsc) and the horizontal scanning line frequency (Fh) for the NTSC, PAL-M, and
standards are as follows: ##EQU1## Thus, for the 525-line NTSC system, there are 227.5 cycles of the subcarrier per horizontal scanning line and, since there are an odd number of horizontal scanning lines (i.e., 525) per an NTSC television frame, exact matching of subcarrier-to-horizontal (SC-H) timing relationship occurs after 1050 horizontal scanning lines or, equivalently, two television frames. In the 525-line PAL-M standard, there are 227.25 cycles of subcarrier per horizontal scanning line and 525 horizontal scanning lines per television frame. Accordingly, exact matching of the SC-H phase relationship for the PAL-M standard occurs after 2100 horizontal scanning lines or, equivalently, four television frames or eight fields. In the 625-line PAL standard, there are 283.7516 cycles of subcarrier per horizontal scanning line and 625 horizontal scanning lines per field. Thus, exact SC-H matching for the PAL standard occurs after 2500 horizontal scanning lines or, equivalently, four television frames or eight fields.
For television signals of the analog type, it is conventional to derive color frame identification information by measurement of the SC-H timing relationship. When making such measurements, the leading-edge of a horizontal sync signal is compared with color burst extrapolated back to the horizontal sync signal. Because such measurements depend upon calibration of signal generation and measuring equipment and are subject to transmission path distortions, complex measurement circuits are required and, even then, the measurements are sometimes ambiguous.
Color frame identification information is needed for various purposes. For example, when a videotape record and playback machine is used to edit color television information as by inserting frames from one recorded television program into another program, it is necessary to know the polarity of the color frames to achieve color coherence and to avoid horizontal picture shifts in edited pictures during playback. In typical editing suites, each video tape recorder has its own color frame detection circuit for making SC-H phase measurements. In practice, such measurements are often inconsistent in editing suites and color frame relationships between video tape recorders are random. Thus, for professional videotape editing and similar purposes, it would be desirable to provide easily accessible color frame polarity information without the need to repeatedly detect color frame polarity by SC-H phase measuring devices.